Quasars and Other Active Galaxies
3C-273 – The brightest Quasar around!
If 100 light years away (~ Pollux in Gemini) it would be as bright as
the Sun is to us now!
3 BILLION solar
masses of
material
Active galaxies are brighter, and emit more nonstellar radiation
In this chapter you will discover…

5-10% of galaxies unusually bright, called active galaxies

The most distant objects we can see: quasars

Supermassive black holes are central engines for radio
galaxies, quasars, Seyfert galaxies, & BL Lac objects

Distribution of Quasars is a key to the evolution of the
Universe, and to the Big Bang Theory of its formation.
Active Galaxy Types

Seyfert Galaxies: Spirals with very bright centers

Radio Galaxies: Ellipticals with huge emissions of
radio energy in “lobes”


Quasars: Very tiny, distant objects
All have very bright, active galactic nucleii
Active Galactic Nuclei
•
Seyfert Galaxies
•
spiral galaxies with
incredibly bright, starlike center (nucleus)
•
they are very bright in
the infrared
Circinus
•
their spectra show
strong emission lines
Active Galactic Nuclei
•
Seyfert Galaxies
The luminosity can vary by as much as the entire
brightness of the Milky Way Galaxy!!
Radio Galaxies
Cygnus A Radio Image
635 Million light years away, and still one of
the brightest radio sources in the entire sky!
Active Galactic Nuclei

Radio Galaxies
|

galaxies – usually giant ellipticals - which
emit large amounts of radio waves

the radio emission come from lobes on either
side of the galaxy; not the galaxy itself
Radio Galaxy Lobes
These lobes are swept back because the galaxy is
moving through an intergalactic medium.
X-ray/Radio Image of Centaurus A
X-ray is blue; radio is red
BL-Lac Objects
Superbright Elliptical Galaxy
Quasars
A peculiar Star-like object, emitting lots of radio
waves? But not with a stellar spectral fingerprint!
Quasar Spectra
Star-like objects
 spectra that look
nothing like a star
 Faint Hydrogen
lines…
 VERY red-shifted!

Quasar Observations

emit light at all wavelengths

A hot dense source?
occasionally VERY strong radio sources
 Associated with jets from galaxies in clusters

Quasars Brightness varies in time!
Quasars Brightness varies in time!
Quasars

Show enormous redshifts  VERY far
away by Hubble’s Law

Show extreme variability  VERY small,
in scales of a light-hours to light years
….and so….
Quasars must be some of the most powerful
objects we know of in the universe!
Quasar Distribution

Seen at greatest distances
(earliest history of the universe!)

Not seen nearby….
But…

Quasar behavior in some nearer clusters
A theoretical model quasar

Must account for observations:
Small Size
 Enormous energy output across spectrum
 Source of Jets
 Similar behavior in galaxies in clusters
 Some radio synchrotron emission (indicating
magnetic field)
 Full spectrum emission

A quasar model…

Supermassive Black Hole Engine

Formed as Galaxies are born…

Pulling in gas, dust, and stars into huge
accretion disk
A quasar model…

Generating jets of X-ray radiation for
millions of years

“Quiet down” as fuel diminishes

“Re-ignited” during collisions & mergers of
galaxies in clusters
Quasar Energy Source?

The energy is generated from matter falling onto
a supermassive black hole…



1.2 x 109 M for NGC 4261
3 x 109 M for M87
…which is at the center (nucleus) of the galaxy.
Quasar Energy Source?
Matter swirls through an accretion disk
before crossing over the event horizon.
 Gravitational energy lost like E = mc2

•

10 – 40% of this is radiated away
Process is very efficient
A quasar model…

Works to explain quasars
Even Supermassive Black holes are TINY
 Accretion Disk generates thermal spectrum,
jets, magnetic fields
 Highly variable as mass is pulled in

and…
 Works for active galaxies, too!
Theory
Observation
Quasar Jet Formation

Magnetic fields twisted

Pull charged particles
out of disk & accelerate
like slingshot

Particles bound to
magnetic field; focused
in a beam
Model Quasar Accounts for Other
Observations, too

Orientation determines what we see:
•
if beams points at us, see a quasar
•
if not, molecular clouds/dust of galaxy
block view of nucleus
•
we see a radio galaxy or Seyfert
•
lobes are where jets impact
intergalactic medium
Hubble space telescope shows us that
quasars do live in galaxies…they are
Active Galactic Nuclei!
If the theory is right --- all galaxies start
with Black Holes!
Evidence Quasars are distant?
Hubble’s Law
 Association with Galaxies in clusters
 Gravitational Lensing

Essay Questions to Know!

What are active galaxies? How do active
galaxies produce their energy? How do
we know?

What are quasars, and where are they
found? What do they tell us about the
universe?